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Step-by-Step Guide to extracting image data

This page describes the typical steps a user might go through when extracting data using the image extraction form. It is aimed at the first-time user and goes on to describe how the data can be displayed and interrogated using the GAIA software package.

The form under discussion allows the user to extract up to a 15 arcmin area from SuperCOSMOS scans of various Schmidt photographic surveys. If the requested part of the sky is on-line a pixel image (FITS format) with an embedded object catalogue (FITS table) is returned.

  • Step 1 - assuming you know the RA (hours minutes seconds) and Dec (degrees, minutes, seconds) of your object for either equinox B1950 or J2000 you can proceed directly to the form located at Packages such as coco allow you to convert between various equinoxes and coordinate systems (galactic, ecliptic etc).

  • Step 2 - fill in the on-line form using the boxes and buttons provided.

    Coordinates: The form should ignore all input characters apart from numbers and + or - and replaces them with blanks. i.e if you wanted to enter the RA and Dec as 01:20:30.7 -30:40:50 the form will work but it is really expecting 01 20 30.7 -30 40 50 so to avoid mis-interpretation try and stick to this format.

    Equinox: Select J2000 (default) or B1950 from the menu.

    Size: Enter a value for the size of the box to be extracted; 0.1 < size < 15 arcmin

    Survey/Waveband: Select which Schmidt survey or waveband you want from the menu. The ESO and POSS-I red surveys were measured to provide a first epoch. The UKST red survey provides a more recent epoch. The on-line availability of the various surveys can be found under sky coverage.

    GIF: If you wish a GIF image of the area of sky requested click the circle next to "yes" to over-ride the deafult "no". Producing the GIF file takes a bit more time. (A FITS file is always returned, once downloaded FITS files can readily be converted to GIFs, JPEGs etc, eg using "xv".)

    Once the form is filled in click on "send". An example of a filled-in form as a screen capture is given here.

  • Step 3 - if the requested part of the sky is up on disk you should see a results page something like this. The GIF image (if requested) is displayed at 200x200 pixels, click on the image to view it at its full resolution.

    As well as links to the gzipped FITS file and GAIA listing the form also returns a link to a data guide.

    If the scanned data has not yet been put on-line or you have requested a survey that does not cover the coordinates requested then the form should return an "error" message. Similarly if the form cannot handle the data you have inputted (eg if the size is larger than 15 arcmin) then you will be informed. If you think you have entered sensible parameters and the charts of scanned data (located here) indicate the survey field/plate has been put on-line then please email with the search parameters used and the messages returned.

  • Step 4 - To save the gzipped FITS image to your local disk space click (in Netscape use shift + mouse button 1, in Internet Explorer use the right mouse button + `Save target as') on the `FITS.gz' button. Some browsers are configured so that the FITS file is automatically uncompressed on download. If the file is saved in the compressed format then use the Unix command gunzip to uncompress it. (PC/Windows users can use packages such as Winzip to uncompress files)

    If you wish to save the GAIA listing of the object catalogue to disk then `click' on the `Tab' button as described in the previous paragraph.

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